1. Field of the Invention
The present invention relates to charged-particle beam processing. In particular, the present invention relates to methods of controlling milling of a specimen using a charged-particle beam such as a focused-ion beam (FIB).
2. The Prior Art
IC devices having three to five or more metal layers are increasingly common. With planarized devices, underlying layers are hidden and buried conductors are difficult to locate. Power planes can cover large areas of the IC, especially with advanced, highly integrated logic devices such as microprocessors. FIG. 1a illustrates in cross-section (not to scale) a portion of such a device having a silicon substrate 100 with diffusion regions 110, 120, a first metal layer M1, a second metal layer M2, a third metal layer M3, a fourth metal layer M4, dielectric 130, and passivation 140. Layer M4 is a power plane under which are hidden conductors or layers M3, M2 and M1.
It is frequently necessary to cut windows in top-layer power planes to expose lower-level signal conductors for probing (e.g., with electron-beam and/or mechanical probes) and/or for making repairs (e.g., with FIB and/or laser systems). It is also sometimes desired to mill a hole down to a metal layer, stopping the milling when the metal is exposed in the desired area. FIG. 1b illustrates an idealized window 150 (not to scale) cut through passivation 140, layer M4 and dielectric 130 to expose a conductor 160 of the device of FIG. 1a.
A FIB is commonly used for these purposes, despite the uneven surfaces often produced. Holes or pits result from preferential etching or milling at varying rates. These can be caused by edges in surface topography, inhomogeneities in underlying layer(s), differing crystal orientations or grain structures in the material being milled, and other factors. The surface inside or under the window becomes extremely rough and can in some cases limit or prevent further operations or repair. Variations from device fabrication process to process and even from device to device can be extreme. Milling of tungsten can be especially difficult to manage because tungsten mills more slowly than other materials and because different grain orientations appear to mill at different rates. Adequate windows can often be milled if the operator is sufficiently skilled, but sometimes the pitting effect is so extreme that further milling operations become difficult or impossible. While a skilled operator may be able to cope with the uneven milling (for example, by suitably tilting the sample), it would be preferable to avoid the need for a skilled operator.
FIG. 1c show an example of a problem which can arise with FIB milling. As passivation layer 140 is milled, the surface 170 of the milled passivation becomes pitted and uneven. FIG. 1d illustrates a typical result as FIB milling continues through layer M4. The metal at the bottom of the window 180 is partially milled through, leaving islands of metal amid regions where dielectric 130 is exposed.
FIG. 2 is a FIB image of portion of a device which has been sectioned. The image has been enhanced with solid lines to emphasize materials transitions. Visible in FIG. 2 are silicon substrate 210, a silicon oxide layer 215, a metal-2 layer 220 of tungsten, a metal-3 layer formed as a sandwich of tungsten 225 and aluminuna 230, a silicon oxide layer 235, Si.sub.3 N.sub.4 passivation 240, tungsten vias 245, 250 and 255 between the metal-2 and metal-3 layers, and silicon oxide regions 260, 265, 270 and 275. The aluminum 230 and tungsten 225 of the metal-3 layer are lumpy and of uneven thickness, not conducive to milling an ideal window for exposing a conductor of the metal-2 layer.
FIG. 3 is a FIB image of the device of FIG. 2 after a FIB has been used to begin milling a window 300 of about 20.times.25 microns. The passivation 280 is visible in the area surrounding window 300. Within the window, the brightest areas 310 are aluminum, the darker gray areas 320 are tungsten, and the darkest areas 330 are dielectric visible where the FIB has cut entirely through the metal-2 layer. The edge of the window appears as a bright line 340.
FIG. 4 is a FIB image of the device of FIG. 3 after further FIB milling of window 300. The device is tilted to better show edges of the window and the milled surface. The unmilled surface of passivation layer 280 is rough, unlike the idealized drawings of FIGS. 1a-1d. Within window 300, craters and pits extend down to the (dark) dielectric material below the (bright) metal layer, while islands of the metal-3 layer remain. Edges of window 300 and edges around the craters appear bright. Preferential milling of edges has made the milled surface pitted and uneven.
Repair of a device sometimes calls for FIB milling to cut a conductor. FIG. 14a shows an idealized schematic section (not to scale) of a portion 1400 of an IC having a dielectric layer 1405 below and a passivation layer 1415 covering dielectric layer 1405 and a metal-2 power bus conductor 1410. FIG. 14a also shows a substrate 1420, a dielectric layer 1425 and a metal-1 conductor 1430. FIG. 14b is a top view showing a portion of the IC much as it would appear in a FIB image, with the edges of conductor 1410 visible as topographical contrast in the image along lines 1435 and 1440. If it is desired to isolate the power bus by cutting conductor 1410, the FIB-system operator defines a "cut box" on a displayed FIB image to establish the boundaries of a region to be milled. Such a "cut box" is shown at 1445 in FIG. 14b. FIB milling is then controlled to mill only the region within the "cut box", that is, by scanning a FIB 1450 between scan limits 1455 and 1460 as shown in the sectional view of FIG. 14c.
A typical result of such a milling operation is also shown in FIG. 14c. Topographical relief in passivation layer 1415 at the edges of conductor 1410 lead to preferential milling, resulting in deep pits 1465, 1470 at each end of the milled region while conductor 1410 is not yet milled through. FIG. 14d is a top view showing a portion of the IC much as it would appear in a FIB image, with the edges of the milled region visible as topographical contrast along lines 1475 and 1480 and with the steep walls of pits 1450 and 1455 visible in the image as topographical contrast and partially as materials contrast at regions 1485 and 1490, respectively. Continued milling in this fashion may extend pits 1450, 1455 into other structure and possibly damage the IC.
FIG. 15 is an actual FIB image of a trench milled to cut a power bus using such a prior-art method of FIB milling. The image is enhanced with inked lines to emphasize contrast features. Visible at 1505 and 1510 are regions of passivation overlying a conductor. Edges of the milled trench are visible as contrasting areas at 1515 and 1520. Unwanted, deep pits at each end of the milled trench are seen at 1525 and 1530. A technique employed by some FIB-system operators to reduce the amount of pitting at the ends of the trench is to begin milling, then acquire and examine a contrast image of a region including the trench, then manually reduce the boundaries of the "cut box" to avoid deepening the pits during further milling. Among other drawbacks, such a conductor-cutting operation is time-consuming and the result is dependent on the skill of the operator.
Chemically-assisted ion beam etching (CAIBE) can sometimes be used to improve the quality of the milled surface. With CAIBE, a jet of a suitable chemical is directed at the surface location where the ion beam is applied. However, CAIBE of uneven surfaces or composite structures can result in "islands" of material projecting from the milled surface.